Final answer:
About 50-60% of an adult's body weight is comprised of water, with this percentage varying across the lifespan and among different body components. The brain and kidneys have the highest water content, and maintaining hydration is vital for several bodily functions.
Step-by-step explanation:
The percent of body water varies throughout the human lifecycle. For instance, infants have about 75% of their body mass as water, while adults have about 50-60% of their body weight as water, which may decrease to around 45% in old age. This variation is due to changes in the proportions of organs, muscles, fat, bone, and other tissues as one matures from infancy to adulthood. The brain and kidneys are composed of 80-85% water, showcasing the highest water proportions in the body, whereas the teeth contain just 8-10% water, the lowest proportion.
Water is not only present within cells but also in the spaces between them, making up tissues and organs. It plays a critical role in numerous bodily functions, including digestion, circulation, nerve signaling, temperature regulation, and the elimination of bacteria and viruses. On average, an adult loses about nine liters of water daily through processes like urination, perspiration, and breathing. Therefore, it is commonly recommended to drink at least five 16-ounce glasses of water or other noncaffeinated beverages each day to stay hydrated.
Blood, which is about 8% of an adult's body weight, primarily consists of plasma, which in turn is made up of about 92% water. This exemplifies the ubiquity and importance of water in the body's various components and systems.